In liquid chromatography and adsorptive separation in general, the fluid distribution system is of paramount importance to the overall performance, particularly for columns with large cross-sections in relation to bed height.
Columns for liquid chromatography normally comprise a vessel filled with a porous bed through which a liquid flows, with separation taking place by material distribution between the liquid and the solid phase of the porous bed. The porous bed is commonly a packed bed, typically formed by consolidating a suspension of discrete particles, but it can also be a monolithic porous body or a stack of porous sheets (membranes). The column often has a cylindrical geometry, with flow passing axially from one end to the other, but columns with radial flow are also well known and non-cylindrical geometries have been described. In all these constructions, the liquid flow must be well distributed from a feed tube over the entire bed surface. As the scaling parameter of chromatography columns is usually the column diameter, with the bed height kept constant, the difficulty of distribution is considerably higher for large-scale columns with correspondingly low height-to-diameter ratios.
A uniform flow distribution is essential in order to obtain good efficiency for the column. Uniform flow distribution is a prerequisite for achieving uniform residence time for all fluid elements passing the packed bed and column, respectively. Any deviations from uniformity will show up as premature breakthrough, low plate numbers or peak asymmetry as they generate an unfavourable broadening in the residence time distribution over the column. Two features of a distribution system are essential for achieving uniform flow distribution: The first feature is the ability of the distribution system to transfer fluid from essentially a single tubing feeding liquid to the column onto the surface of the packed bed such that all fluid elements are applied simultaneously over the packed bed surface. The same simultaneous withdrawal and collection of fluid applies to the removal of fluid at the column outlet. The second critical feature is the ability to maintain uniform pressure across the surface of the packed bed which is yielding a uniform fluid velocity over the bed and the column.
A classical fluid distribution system for axial columns simply consists of a central inlet for the mobile phase in combination with a thin distribution channel (gap) of constant height behind a retainer filter (woven net or sinter) confining the inlet end of the bed. This type of system will by necessity deteriorate strongly in performance with increasing diameter of the column. This is due to the residence time difference between fluid elements travelling from the inlet to the outer column wall and those fluid elements which directly can enter the retainer net and the bed region below the inlet port. Further, the required fluid transfer of liquid throughout the distribution channel towards the column wall will result in a pressure drop across the distribution channel. As a result, the pressure drop over the packed bed and thus the uniformity of the fluid velocity field will be affected.
A better fluid distribution will be provided with a conically shaped flow channel with the largest channel height at the position of highest fluid velocity to balance the volumetric flow in the path. Such conical flow channels have been described in e.g. U.S. Pat. No. 5,137,628 as an open channel below one or two nets or in U.S. Pat. No. 6,936,166 as a pattern of channels in a ribbed plate below a retainer net and a perforated plate. The ribbed plate has the disadvantage of high production cost and complicated design engineering. In the open channel case, there is a high risk for bulging of the retainer net when it is subjected to a) hydrodynamic forces during operation that are counteracting the pressure loss over the porous bed and b) forces from the mechanical compression of the packed bed as result of the packing process and the weight of the bed. Any such bulging will affect the performance of the column negatively. This adverse effect on the column performance is due to volumetric changes of the packed bed geometry as a result of the bulging. These volumetric changes will cause instability as well as inhomogeneous compression and porosity in the packed bed structure. Further, bulging of the retainer net will have an adverse effect on the column performance by reducing and altering the overall volume and dimensions of the distribution channel, hereby leading to changes in fluid velocity and pressure loss along the distribution channel that can strongly deteriorate the overall residence time distribution and performance of the column.
In many cases it is desirable to use only plastic materials in the distributor. This applies both to columns for use with liquids that will corrode stainless steel and to lower cost columns intended for single or short-term use. Plastics have a lower elastic modulus than steel, which puts higher demands on the retainer net support arrangements in order to prevent bulging. It will also be more important to keep manufacturing costs low in the plastics case when a column and distributor design is needed for producing single-use columns and distribution systems that are to be disposed of after a process run or a campaign.
There is thus a need for a low cost distributor giving uniform flow distribution and no retainer net bulging during operation of the column.